Supportive housing (SH) is a structural intervention that is being increasingly applied to homelessness among transition aged youth (TAY). Despite data indicating the positive effects of SH on housing stability, there are elements of SH that may promote risky behaviors that increase HIV rates. This study investigates the mechanisms that explain the association between SH and HIV risk behaviors. In addition, since research has not addressed whether SH yields the same outcomes for all TAY, we will investigate how individual factors such as age, gender, or sexual orientation moderate the effects of SH on TAY HIV risk behaviors. The goal of this investigation is to provide an empirical basis to guide the future development of SH for homeless TAY. This 3-year mixed methods study is guided by a robust social ecological conceptual framework of risk environment that refers to a space- whether social and/or physical-in which a variety of individual and contextual factors interact to influence HIV risk behaviors. For aim 1 (quantitative), we examine the impact of SH on TAY HIV risk behaviors (sexual risk and substance use) by recruiting 150 TAY living in SH along with a matched comparison group of 150 TAY who are currently homeless (total N=300). Ecological momentary assessment (EMA), which is considered the gold standard for capturing ecologically valid self-report, will be used to collect context specific predictive data on HIV risk behaviors. TAY will be prompted multiple times a day over a one week period to answer a series of surveys using mobile phone technology. For aim 2 (qualitative), we will investigate high and low risk environments within SH for TAY through the use an interactive, personalized geospatial map rendering of EMA responses (generated through mobile phone GPS) as an elicitation device for in-depth interviews. 60 participants will be purposively sampled to represent both high (n=30) and low (n=30) HIV risk taking. For aim 3, we will develop recommendations for designing SH for TAY that minimizes HIV risk though guideline development methods adopted by the World Health Organization. In addition to providing an empirical basis to guide the future development of SH for homeless TAY, the findings from this study can capitalize on TAY's widespread use of mobile phone technology and be used to design Ecological Momentary Interventions for both homeless and housed TAY.